The invention relates to optical fiber cabling. More particularly, the invention elates to optical fiber cables apparatus having stacked ribbon fibers therein, and systems using such apparatus.
Optical cables typically comprise one or more multi-fiber units such as a multi-fiber unit tube. A multi-fiber unit tube typically contains a plurality of individual optical fibers disposed therein. In some configurations, the optical fibers are arranged loosely within the multi-fiber unit tube. See, e.g., U.S. Pat No. 5,751,880. However, such arrangements have relatively low fiber packing densities and many applications desire optical fiber cables that have higher fiber packing densities. See, e.g., U.S. Pat No. 5,155,789, in which the optical fiber cable comprises a plurality of supporting sheaths each tightly bound around a plurality of optical fibers.
Alternative optical fiber cable configurations include multi-fiber unit tubes having one or more optical fiber ribbons disposed therein. The optical fiber ribbons typically are stacked within the multi-fiber unit tube to form what is referred to as a ribbon stack or optical fiber ribbon stack. See, e.g., U.S. Pat Nos. 5,857,051 and 5,229,851.
Many conventional optical fiber ribbon stack arrangements include various kinds of filler materials, such as hydrophobic (water blocking) and hydrophilic (water absorbing) gels, powders, yarns and tapes. For example, see U.S. Pat No. 6,035,087, which discloses an optical fiber cable that uses a hydrophobic gel 7 (see, e.g., FIG. 3), and U.S. Pat No. 4,867,526, which discloses an optical fiber cable that uses a hydrophobic tape between the transmission media and the outer jacket of the cable. Also, e.g., see U.S. Pat No. 5,751,880, which discloses an optical fiber cable that uses a hydrophilic water swelling material.
Also, most conventional optical fiber cable configurations include a strength member, often located centrally within the cable and surrounded by a plurality of multi-fiber unit tube, or embedded or otherwise formed in one of the cable layers (i.e., in a protective sheath surrounding the multi-fiber unit tubes. For example, see U.S. Pat. No. 5,857,051 (FIG. 4), which is assigned to the assignee of this application, U.S. Pat Nos. 5,229,851, and 5,531,064 (FIGS. 1 and 5). However, also see U.S. Pat. No. 4,859,023, which is assigned to the assignee of this application, which discusses a method for making an optical fiber cable that does not include compressive strength members.
It would be desirable to have available optical fiber cables, including ribbon-stacked optical fiber cables, with improved optical fiber densities. Also, it is desirable for such optical fiber cables to not rely on dedicated strength members disposed therein for overall tensile strength and crush resistance.
The invention is embodied in an optical fiber cable having improved optical fiber densities and no central strength member. The optical fiber cable includes one or more multi-fiber unit tubes having an optical fiber ribbon stack snugly positioned therein. According to embodiments of the invention, the diagonal length of the ribbon stack is approximately equal to the inner diameter of the multi-fiber unit tube or, alternatively, the ratio of the ribbon stack diagonal length to the multi-fiber unit tube inner diameter is at least 0.90. The multi-fiber unit tube is made of a material suitably soft and flexible to allow the ribbon stack to be relatively firmly positioned within the multi-fiber unit tube without compromising the optical performance of the fibers in the ribbon stack. The multi-fiber unit tube is made of, e.g., low-density polyethylene (LDPE), and has a modulus less than approximately 70,000 pounds per square inch (psi). The ribbon stack comprises any suitable number of optical fiber ribbons stacked to form an array of fibers, e.g., a 12-fiber stack formed by 3 4-fiber ribbons stacked together, or a 144-fiber stack formed by stacking together 12 12-fiber ribbons. Also, the optical fiber cable includes one or more filling materials such as stranded yarn fillers positioned, e.g., between the ribbon stack and the multi-fiber unit tube, to maintain the shape of the multi-fiber unit tube. Alternatively, the yam filler material is impregnated with otherwise includes super absorbent powder (SAP).